Salmonella pathogens infect over a billion people each year worldwide resulting in 3 million deaths annually from septicemia, mostly in HIV-infected patients and 700,000 from typhoid fever (W.H.O. estimates). Salmonella enterica alternatively expresses one of two antigenically distinct flagellin proteins, FliC or FljB. The flagellar filament proteins are major targets of the host immune system, but the role of flagellar phase variation in Salmonella pathogenesis is unclear. The Hin recombinase catalyzes a site-specific DNA inversion event in the S. enterica chromosome that controls the alternative expression of flagellin antigens. Upon exposure to HeLa cells, the Hin recombinase is secreted into the extracellular medium and also translocated into the HeLa cells where it is localized into the cell nucleus. Secretion of Hin requires the flagellar Type III secretion system, whereas translocation requires both the flagellar and SPI 1 Type III secretion systems. Also, proper regulation of flagellar genes is essential for Salmonella pathogenesis. The research proposed here will examine the effect of Hin secretion/translocation and Hin-mediated flagellar phase variation on Salmonella pathogenesis, the mechanism of Hin secretion and translocation by the flagellar and SPI 1 Type III secretion systems, the effect of Hin secretion on the regulation of flagellar phase variation, and the effect of the different flagellins on invasion, liver and spleen colonization. In this proposal we will also address the host response to Hin translocation. A particular focus of this grant will be to characterize the role of the Hin recombinase in S. enterica pathogenesis and to elucidate the mechanism of Hin secretion and translocation. [unreadable] [unreadable]